The Collaborative International Dictionary of English v.0.48:

Endothelium \En`do*the"li*um\, n.; pl. Endothelia. [NL., fr.
   Gr. 'e`ndon within + ? nipple.] (Anat.)
   The thin epithelium lining the blood vessels, lymphatics, and
   serous cavities. See Epithelium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

endothelium
    n 1: an epithelium of mesoblastic origin; a thin layer of
         flattened cells that lines the inside of some body cavities